Preparation of carbon fibers from polyvinyl alcohol base fibers



United States Patent 3 488,151 PREPARATION OF CARBON FIBERS FROM POLY-VINYL ALCOHOL BASE FIBERS William J. Noss, Marshallville, Ohio, assignorto Union Carbide Corporation, a corporation of New York No Drawing.Filed Sept. 1, 1967, Ser. No. 664,953

Int. Cl. C01b 31/07 US. Cl. 23-2091 11 Claims ABSTRACT OF THE DISCLOSUREA process is provide for producing a high weight yield of carbon fibers,preferably in a textile form, from thermoplastic polyvinyl alcohol basefibers. This process comprises converting a thermoplastic polyvinylalcohol base fibrous starting material to a thermoset polyvinyl alcoholbase fibrous material by subjecting it to the action of a chemicaloxidizing agent selected from the oxygen containing compounds of themetallic transition elements and then carbonizing the resultantthermoset fibrous polyvinyl alcohol base material by heating it in aninert atmosphere to a temperature in excess of about 700 C. for a timesufiicient to produce a substantially all-carbon base fibrous productwhich retains the physical characteristics of the starting fibrousmaterial, such as hand and drape.

BACKGROUND OF THE INVENTION Field of the invention The present inventionrelates to an improved process for the production of carbon fibers,preferably in textile form, from polyvinyl alcohol base fibers andtextiles. As used herein and in the appended claims, the term carbon isintended to include both the non-graphitic and graphitic forms ofcarbon.

Description of prior art Carbon is an element which possesses manyinteresting and useful chemical and physical properties. It is amaterial which both can be found in nature and produced synthetically.Carbon is a readily processible material and can be fashioned intoalmost any intricate shape or pattern. Today, the uses of carbon incommerce and industry are myriad.

Presently, most of the carbon articles used in industry are produced bya process which comprises mixing nongraphitic carbon particles with acarbonizable binder, extruding or molding the so-produced mixture intothe desired shape or article and, subsequently, heating it to atemperature sufficient to carbonize the binder phase. If, during thisheating the maximum temperature which the resultant article experiencesis of the order of 700-900 C., it is said to be a non-graphiticall-carbon article. However, if the article is further heated to atemperature of the order of 20002500 C. and higher, it is said to beconverted to a graphitic form of carbon and is generally calledgraphite.

Recently, there has been introduced to the carbon art a fibrous form ofcarbon. This form of carbon is unique in that it possesses theflexibility of a fibrous material while at the same time ischaracterized by the electrical and chemical properties associated withconventionally formed carbon articles.

United States Patents 3,011,981 which issued Dec. 5, 1961 to W. T.Soltes describes and claims a method for manufacturing carbon in atextile form. Briefly, the process disclosed therein comprises heating acellulosic textile in an inert atmosphere at a progressively highertemperature until substantial carbonization of the textile occurs. Theresultant product possesses the chemical and physical attributesexhibited by conventionally formed carbon articles while it retains theflexibility and other physical characteristics associated with thetextile starting material, such as hand and drape.

A textile form of fibrous graphite is disclosed and claimed in UnitedStates Patent 3,107,152, which issued to C. E. Ford and C. V. Mitchellon Oct. 15, 1963. Broadly stated, the process for producing fibrousgraphite disclosed therein comprises heating a cellulosic startingmaterial in an inert atmosphere at progressively higher temperatures forvarious times until a temperature of about 900 C. is achieved followedby further heating in a suitable protective atmosphere at still highertemperatures until substantial graphitization occurs. The productproduced by this process exhibits the chemical and physical propertiesgenerally associated with conventionally fabricated graphite while, atthe same time, it retains the textile characteristics of the startingmaterial.

In addition to the foregoing methods for producing carbon textiles,recently there has been introduced to the carbon art a method forproducing carbon textiles from polyvinyl alcohol base textiles. Briefly,this method comprises heating a thermoplastic polyvinyl alcohol basetextile in an atmosphere which contains uncombined gaseous oxygen toconvert it to a thermoset material which can then be subjected to acarbonizing treatment without destroying the textile structure evidencedby the starting material. However, when this technique is employed thetime required to convert the thermoplastic polyvinyl alcohol basematerial to a thermoset product is exceptionally long and commerciallyimpractical. In fact, even at the optimum temperature of 200 C. thegaseous oXidati n treatment must be continued for from 16 to 24 hours inorder to obtain, on subsequent pyrolysis, a carbon fiber in reasonableyields which exhibits adequate physical properties.

The foregoing disadvantages are readily overcome by the practice ofapplicants process for producing carbon textiles from polyvinyl alcoholbase textiles. The benefits obtained from the use of his invention willbe discussed later in more detail. However, it should be noted at thistime that by the use of applicants technique for pro ducing carbontextiles it is possible to reduce the time required to convert athermoplastic polyvinyl alcohol base textile material to a carbonizablethermoset polyvinyl alcohol base textile material from in excess of 16hours to less than about 0.5 hour.

SUMMARY Briefly, the subject invention is accomplished by a processwhich comprises the steps of subjecting a thermoplastic polyvinylalcohol base fibrous material, preferably in textile form, to the actionof a chemical oxidizing reagent selected from the oxygen containingcompounds of the metallic transition elements for a time sufficient toconvert the thermoplastic polyvinyl alcohol base fibrous material to anessentially thermoset material which can then be converted to a carbonfibrous material by heating it in a non-oxidizing atmosphere at atemperature in excess of 700 C.

The principal advantage of the instant invention is that it eliminatesthe lengthy and uneconomical gaseous oxidation treatment heretoforerequired to be employed when one is desirous of converting polyvinylalcohol base fibers and textiles to a fibrous form of carbon.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION Polyvinylalcohol base fibers and textiles contemplated for use as startingmaterials in the practice of the present invention are those which havebeen fashioned from polyvinyl alcohol base polymers that have beeneither chemically treated or cross-linked to render them insoluble inwater. One well known technique for controlling the solubility ofpolyvinyl alcohol base polymers is to react them with materials such asformaldehyde; another is to crosslink the polyvinyl alcohol structure byheating such polymers to a temperature of about 250 C.

Illustrative of the type of polyvinyl alcohol base polymeric materialswhich may be employed in the practice of the present invention arehomopolymers of polyvinyl alcohol; copolymers of vinyl alcohol and vinylacetate; terpolymers of vinyl alcohol, vinyl acetate and maleicanhydride. It is not intended to limit the present invention to theforegoing types of polyvinyl alcohol base polymers for other Well knownpolyvinyl alcohol base polymeric materials will be readily apparent tothe skilled artisan.

Any chemical oxidizing agent or solution containing an oxygenatedcompound of the metallic transition elements is suitable for thepractice of the invention. However, best results are obtained when thetransition metal element is selected from the group consisting oftitanium, vanadium, chromium, molybdenum, manganese, tungsten, cobalt,platinum and palladium. The preferred oxidizirig reagents or compoundsare those which contain a transition metal in its highest oxidationstate. In particular, two chemical reagents which have been found to beespecially suited to the practice of the invention are potassiumdichromate and chromium trioxide.

Preferably, the transition metal containing oxidizing agent is appliedto the polyvinyl alcohol base fibers and/ or textiles in solution form,i.e., it is dissolved in a suitable vehicle. In practice, this vehiclemay be acidic, basic or neutral. All that is required is that it doesnot react with the transition metal dissolved therein so as to destroyits effectiveness as a reagent which can convert a thermoplasticpolyvinyl alcohol base material to a thermoset polyvinyl alcohol basematerial which will carbonize without losing its original textilecharacteristics.

In addition to the foregoing technique for practicing the instantinvention, it should be noted that it is oftentimes desirable to employthe transition metal containing oxidizing agent in the melt or vaporstate for all that is required is that it be brought into direct contactwith the material being treated.

The temperature at which a polyvinyl alcohol base textile material maybe advantageously subjected to the action of a transition metal-oxygencontaining reagent according to the teachings of the instant inventionvaries from below room temperature to over 200 C. The effect ofincreasing the treatment temperature is the usual one of increasing therate of reaction. It should again be noted that the invention is mostefficiently and effectively carried out when the transition metalelement in the transition metal-oxygen containing solution is present inits highest oxidation state. For example, potassium permanganate is muchmore effective than manganese dioxide since in potassium permanganatethe manganese is in the +7 oxidation state and +4 in the manganesedioxide.

The present invention will now be described in greater detail in thefollowing examples.

Example I An approximately 1 gram batch of homopolymer polyvinyl alcoholbase fibers in yarn was washed in a solution oftrifluoro-trichloroethane for about 10 minutes to remove any surfacefinish therefrom. In their manufacture, these fibers had been treatedwith a formaldehyde solution to render them insoluble in Water. The yarnwas then removed from the cleansing solution and vacuum dried for 20minutes at 30 C. The so-cleansed yarn consisting of 36 monofilaments,each having a denier of about 6.7, was placed into a vessel whichcontained an acidified 1 percent solution of chromium trioxide. Thissolution was prepared by adding 1 gram of chromium trioxide to 98 ml. ofa .05 molar aqueous solution of sulfuric acid. The temperature of theoxidizing solutign maintained at '4 approximately C. The thermoplasticpolyvinyl alcohol homopolymer base yarn was allowed to remain in theheated chromium trioxide containing oxidizing solution for 5 minutes inorder to convert it to an essentially thermoset material, i.e., amaterial which could be carbonized without fusing. The resultant yarnwas removed from the oxidizing bath, washed with water for 10 minutes,rewashed with a 5 percent solution of sodium hydrosulfite until freefrom residual oxidizing material and, subsequently, vacuum dried forabout 1 hour at 60 C. The oxidized yarn was then carbonized in an argonatmosphere at a rate of 60 C. per hour to 1000 0, followed by a 1 hourhold at this temperature. The resulting weight yield of carbon was 31.5percent. The average tensile strength of the yarn monofilaments was26,800 pounds per square inch. The Youngs modulus of elasticity was 2.710 pounds per square inch. The resultant carbonized yarn exhibited thetypical chemical and physical properties of non-graphitic carbon whileit retained its original textile characteristics, such as hand anddrape.

Example II The procedure described in Example I was repeated except thatapproximately 1 gram of chromium trioxide was dissolved in 98 ml. of anaqueous. 1 M nitric acid solution and the duration of the oxidationtreatment therein changed to 10 minutes. Upon carbonization, theresulting weight yield of carbon was 30.1 percent. The average tensilestrength of the yarn monofilaments was 31,000 pounds per square inch.The Youngs modulus of elasticity was 2.3 l0 pounds per square inch. Theresultant non-graphitic carbon yarn exhibited the typical chemical andphysical properties of non-graphitic carbon while it retained itsoriginal textile characteristics, such as hand and drape.

Example III The procedure described in Example I was repeated exceptthat the oxidizing solution was prepared by dissolving 1 gram ofpotassium dichromate in 98 ml. of .05 M aqueous sulfuric acid solution.After carbonization, the resulting weight yield of carbon was 29.8percent. The average tensile strength of the yarn monofilaments was16,600 pounds per square inch and the Youngs modulus of elasticity was2.7 10 pounds per square inch. The resultant non-graphitic carbon yarnexhibited the typical chemical and physical properties of non-graphiticcarbon while it retained its original textile characteristics, such ashand and drape.

Example IV A .9056 gram polyvinyl alcohol yarn specimen Was positionedinside of a reaction vessel and exposed under refluxing conditions tothe oxidizing action of vaporized chromyl chloride. The temperature ofthe vapor was approximately 118 C. The duration of the oxidationtreatment was about 15 minutes after which the yarn was withdrawn fromthe apparatus and water washed to remove any residual chromyl chloride.The yarn was then vacuum dried at 60 C. for one hour. The resultantoxidized thermoset yarn was carbonized by heating it under non-oxidizingconditions at a rate of 60 C. per hour to 1000 C. followed by a hold atthis temperature for 1 hour. The yarn so-produced exhibited the chemicaland physical properties of carbon while it retained its original textilecharacteristics such as hand and drape. The weight yield of theso-treated and carbonized fibers was 15.1 percent.

While the foregoing examples all concern the use of a polyvinyl alcoholhomopolymer base yarn, which is preferred, it will be readilyappreciated by those skilled in the art that other yarns and textilescontaining less than 100 percent vinyl alcohol material, as hereinbeforedescribed, are also amenable to the practice of the inven-. on I d t ithas be n d sco ered that p r results are obtained when the filamentswhich make up the yarn or textile used in the practice of the inventionhave an extremely small cross section or diameter and are free of anysurface finish. Also, it has been observed that highly desirable resultsare achieved when one uses a dilute solution of a strong oxidizingreagent such as potassium dichromate or chromium trioxide as opposed toa highly concentrated one. The preferred oxidizing temperature in thepractice of the invention is that at which only a few minutes residencetime in the oxidizing solution is required to effect the transformationof polyvinyl alcohol base material from a thermoplastic to a thermosetmaterial which is amenable to carbonization at elevated temperatureswithout losing its original textile characteristics. It will beappreciated by those skilled in the art that the lower the bathtemperature and the more dilute the oxidizing solution, the longer willbe the residence time required to produce the necessary degree of fiberoxidation.

In addition, it should be noted that if a non-aqueous oxidizing solutionis employed, the polyvinyl alcohol base fibers must be such that theyare not soluble in the nonaqueous media. Polyvinyl alcohol base fibersof this type are well known in the art and are readily availablecommercially.

The foregoing information and examples are presented herein forillustrative purposes only and are not intended to unduly limit thescope of the invention.

What is claimed is:

1. A process for the manufacture of a carbon fiber from a thermoplasticpolyvinyl alcohol base-fiber comprising the steps of:

(a) oxidizing said thermoplastic polyvinyl alcohol base fiber bycontacting it with an oxygen containing compound of the transition metalelements for a time sufficient to convert said thermoplastic polyvinylalcohol base fiber to an essentially thermoset polyvinyl alcohol basefiber; and

(b) heating said thermoset polyvinyl alcohol base fiber in anon-oxidizing atmosphere to a temperature in excess of 700 C. for a timesufiicient to convert said polyvinyl alcohol base fiber to asubstantially 6 carbon base textile which substantially retains thephysical characteristics of the textile starting material.

2. The process of claim 1 wherein said oxygen containing compound is insolution form.

3. The process of claim 1 wherein said thermoplastic polyvinyl alcoholbase fiber consists essentially of a homopolymer of vinyl alcohol.

4. The process of claim 2 wherein said solution contains a compoundselected from the group consisting of chromium trioxide and potassiumdichromate.

5. The process of claim 4 wherein the duration of said oxidizing step(a) is from 5 to 10 minutes.

6. The process of claim 4 wherein said oxidizing step (a) is carried outat a temperature of about C.

7. The process of claim 1 wherein the transition metal in said oxygencontaining compound of the transition metal elements is selected fromthe group consisting of titanium, vanadium, chromium, molybdenum,manganese, tungsten, cobalt, platinum and palladium.

8. The process of claim 1 wherein said polyvinyl alcohol base fiber isin the form of a yarn.

9. The process of claim 8 wherein the surface of said yarn has beentreated to remove any finish therefrom.

10. The process of claim 1 wherein the transition metal in said oxygencontaining compound of the transition metal elements is in its highestoxidation state.

11. The process of claim 1 wherein said polyvinyl alcohol fiber is intextile form.

References Cited UNITED STATES PATENTS 3,053,605 9/1962 Tanabe et al.8--115.5 3,095,257 6/1963 Tanabe et al. 8--l15.5 3,285,696 1l/l966Tsunoda 23-2091 3,427,120 2/1969 Shindo et al. 23209.2 X

EDWARD J. MEROS, Primary Examiner U.S. Cl. X.R.

